The present invention relates to gravure printing, and more specifically to a screen gravure engraving system for electromechanical engravers.
Recently, with the advent of electronic and computer driven engravers, there has been renewed interest in converting halftone film, including offset halftones, for gravure cylinder application. This is due to the fact that there are major differences in the prepress procedures for offset and gravure printing. In the offset procedure the offset plate is made from four color separation films in the halftone. The color separations for making a gravure cylinder are continuous tone. While the halftone films have only opaque and transparent (black and white) areas, the continuous tone films have a multitude of shades of gray. The halftone films can be corrected by dot etching and can be readily copied by contact exposure. The continuous tone films cannot be corrected as easily and cannot be readily copied. Consequently, continuous tone separations are more costly. Using halftone films reduces the time and cost of prepress operations due to simpler techniques of film copying, film correction and color proofing.
When gravure cylinders are prepared (engraved) by etching techniques, special conversion screens are required. Converlog, Toppan and Neosan are three conversion-by-screen methods. These methods diffuse the light returning from the screen and film separation combination. Depending on the dot area the black and white light is evenly distributed so that the area will look darker or lighter. However, these conversion-by-screen methods are only applicable to the etching of gravure cylinders.
The introduction of the electromechanical engravers, such as the Helio-Klischograph or the Ohio Electronic Engraver Inc., engraver, requires optical scanning of the four color separations point by point or pixel by pixel (picture element by picture element) to form each gravure cell. The scanned digital value for each cell is translated into a penetration depth and the gravure cells are engraved into the gravure cylinder with the proper volume by the stylus of the engraver.
When continuous tone films are being used, the scanner is focused on a single pixel. When halftone or screened films are used, averaging or diffusion must take place. To obtain the same diffusion effect that was achieved with the aforementioned screen methods, defocusing of the scanner's optics is employed. By defocusing, the scanner is made to scan four pixels instead of one. The sensing element in the scanner (photomultiplier) averages the reading. The results produced are somewhat similar to those produced by the aforementioned screen conversion methods.
However, the defocusing method has a major shortcoming in that the edges or other details, where there is a major change in density, do not look sharp; the contrast is reduced and the edges look "soft." This softening at the edges makes the picture look less sharp, less focused. In an attempt to solve this problem, unsharp masking techniques were used including fixed inner and outer circular scanning apertures. The outer aperture reading is utilized to increase the slope between light and dark areas and, therefore, achieve a sharper edge. However, such systems are inflexible and cannot accommodate various screen angles, screen rulings and continuous tone films.